1. Field of the Invention
The present invention relates to a fuel supply apparatus for an internal combustion engine.
2. Description of the Background Art
An internal combustion engine including a first fuel injection valve directly injecting a fuel into a cylinder for stratified charge combustion and a second fuel injection valve injecting a fuel into an intake manifold for homogenous combustion is known.
When fuel injection from the first fuel injection valve is stopped during homogenous combustion in such an internal combustion engine, there is a possibility that a temperature in an injection hole in the first fuel injection valve opening into the cylinder is raised and deposits are produced into the injection hole to cause clogging of the injection hole. In order to avoid such a problem, it has been proposed that not only the second fuel injection valve but also the first fuel injection valve inject the fuel during homogenous combustion, so as to suppress increase in the temperature in the injection hole of the first fuel injection valve (for example, see Japanese Patent Laying-Open No. 2002-364409).
The fuel injected into the intake manifold by the second fuel injection valve is supplied to the cylinder in an atomized manner along with the intake air. Therefore, as compared with the fuel directly injected into the cylinder by the first fuel injection valve, the fuel injected by the second fuel injection valve is advantageous in forming a satisfactory homogenous air-fuel mixture. Meanwhile, when the fuel is injected into the cylinder in an intake stroke by the first fuel injection valve, the temperature in the cylinder is lowered because of latent heat of vaporization of the fuel. Accordingly, knocking can be suppressed and efficiency in charging intake air can be improved.
Therefore, it is considered that an internal combustion engine that does not carry out stratified charge combustion is provided with the first fuel injection valve for directly injecting the fuel into the cylinder and the second fuel injection valve for injecting the fuel into the intake manifold. When the first fuel injection valve is provided as described above, it is preferable to inject the fuel from the first fuel injection valve in order to suppress increase in the temperature in the injection hole also during an operation state not particularly requiring fuel injection by the first fuel injection valve.
In any internal combustion engine, homogenous combustion is performed while an engine is idle. Here, it is preferable to set a ratio of fuel injection from the second fuel injection valve to be larger such that most fuel is injected from the second fuel injection valve advantageous in forming a satisfactory homogenous air-fuel mixture and that the fuel of a quantity just sufficient for suppressing increase in the temperature in the injection hole is injected from the first fuel injection valve.
For stratified charge combustion, the first fuel injection valve should inject the fuel into the cylinder at a high pressure in compression stroke, and therefore, a high injection pressure is set. Even when stratified charge combustion is not performed, the injection pressure in the first fuel injection valve is set high, in order to promote atomization of the fuel to be injected into the cylinder. Accordingly, in the first fuel injection valve, a minimum fuel injection quantity corresponding to a minimum valve-open time period permitting control of a fuel injection quantity is not made so small. In contrast, combustion temperature is not so high particularly during homogenous combustion while the engine is at idle. Accordingly, injection quantity required to prevent production of deposits in the first fuel injection valve is considerably small, and it is even smaller than the minimum fuel injection quantity of the first fuel injection valve. Then, while the engine is idle, the fuel of a quantity larger than necessary is injected by the first fuel injection valve. In addition, as a quantity of fuel necessary for running the engine is also relatively small, a high ratio of fuel injection by the second fuel injection valve cannot be set, which results in difficulty in forming a satisfactory homogenous air-fuel mixture.